Cellulose microfibers are described in U.S. Pat. No. 6,511,746 to Collier et al. In the '746 patent there is described a process for manufacturing cellulose microfibers from dissolved cellulose by extruding the dissolved cellulose (lyocell-type) through a converging die. The fibers are reported to have a diameter on the order of 10 μm with constituent structures of smaller diameters. See Col. 15-16. Note also, U.S. Pat. No. 6,235,392 to Luo et al. which describes melt blown Lyocell microfiber.
Lyocell is made by dissolving nearly pure cellulose in N-methyl-morpholine oxide (NMMO) and reforming fibers by injecting a concentrated cellulose/NMMO solution into a water bath through spinnerets. The water dilutes the NMMO as the nascent fiber is drawn through the bath, and the cellulose crystallizes into fibers. The fiber formation process first produces extremely fine fibrils which then align themselves along the axis of the fiber as the NMMO is removed. The strength of the bonds between fibrils has some distribution around a mean such that mechanical action may completely disintegrate some fibers while leaving others mostly intact. In most textile applications, fibrillation is not desired, and there are patents on ways to minimize fibrillation. In other applications such as filter media, it is desired to retain large fibers with fibrils still attached. In co-pending application U.S. Patent Application Ser. No. 60/850,467 (Case 20134), the lyocell is fibrillated to the point where fibrils are separated into distinct microfibers. FIG. 1 shows an example of fibrillated lyocell.
In the process of generating microfibers via fibrillated lyocell, a number of inefficiencies arise. First, lyocell has a tendency to fibrillate, but the manufacturer employs all means possible to minimize this tendency. Thus, an extended period of low intensity refining is necessary to gently tease the fibers apart. Too much mechanical action will grind and shorten the fibers rather than fibrillate them. Too little mechanical action will reduce the yield of microfibers. Current practice yields about half microfiber and half fibrillated fibers. It may be possible to separate the fractions with screening, but this adds cost. The larger fractions may or may not be readily splittable in any event.
A second inefficiency results from the morphology of the microfiber. Microfibers have extremely low coarseness and are very short. The benefits of microfiber such as bulk, softness, absorbency, wet strength, opacity, and the ability to reduce basis weight are documented in U.S. Patent Application Ser. No. 60/850,467, referred to above. Despite the many benefits of microfibers, it is believed that the tactile benefit and other properties could be improved by increasing the exceptionally low coarseness somewhat and increasing the average length of microfibers in accordance with the present invention.
A third inefficiency arising from conventional lyocell fiber manufacture relates to cost. Wood pulp is purified to alpha cellulose for current practice in order to produce high quality textile fibers. High purity cellulose adds cost. It may be possible and even desirable to use ordinary bleached Kraft pulp for some portion of the raw material. A 6 mm length can be cut on the production line, but shorter lengths require an offline process that adds still more cost. Fibrillation then produces microfibers while leaving some relatively unimpacted fibers. The microfibers so-produced have very low coarseness, but shorter than desired. The lightly impacted fibers are reasonably long, but coarseness is too high. The microfiber is the main desired product of fibrillation, yet the yield of microfiber from fibrillation is perhaps 50%, at best, using conventional techniques.